Today, secondary batteries are broadly used in large devices such as vehicles, power storage systems, etc., and small devices such as mobile phones, camcorders, laptops, etc.
Lithium ion batteries have been mainly used as the secondary batteries due to high energy density and a large capacity per unit area, when compared with nickel-manganese batteries or nickel-cadmium batteries.
However, the lithium secondary batteries are not suitable as next generation batteries for vehicles, since the lithium secondary batteries are easily heated, energy density thereof is approximately 360 Wh/kg, and output thereof is poor.
FIG. 1 illustrates a lithium sulfur battery during charge and discharge according to the related art.
A conventional lithium sulfur battery includes a positive electrode, a lithium negative electrode, and an electrolyte layer disposed between the positive electrode and the lithium negative electrode. The positive electrode includes a positive electrode active material (sulfur), a conductive material, a liquid electrolyte, and a binder.
When the lithium sulfur battery is discharged, electrons migrate to the positive electrode from the lithium negative electrode. When migrating along the conductive material in the positive electrode, the electrons bind to a positive electrode active material (sulfur) adjacent to a surface of the conductive material. The positive electrode active material (sulfur) is reduced to S82−, and S82− forms Li2S8 by binding with lithium ions. Li2S8 continuously reacts with the lithium ions to be precipitated into Li2S2/Li2S on the surface of the lithium negative electrode.
When the battery is charged, an oxidation reaction reversely occurs to form S82−, and S82− loses electrons thereof on the surface of the conductive material to be oxidized to the positive electrode active material (sulfur).
The conventional lithium sulfur battery uses a liquid electrolyte. Since most liquid electrolytes are flammable organic materials, there is a high risk of fire and explosion at high temperature. In addition, some lithium sulfur compounds generated during charge and discharge are dissolved in a liquid electrolyte, and thus, lifespan of a battery deteriorates.
Accordingly, interest for all-solid-state lithium sulfur batteries having a high output and a high energy density has been dramatically increased.
The all-solid-state lithium sulfur batteries use a solid electrolyte instead of the liquid electrolyte, thus preventing the above problems.
Since the all-solid-state lithium sulfur batteries have a theoretical energy density of approximately 2600 Wh/kg that is approximately seven times higher than conventional lithium ion batteries, the all-solid-state lithium sulfur batteries are suitable as a power source for electric vehicles.
In addition, since the all-solid-state lithium sulfur batteries use a solid electrolyte, problems such as electrolyte leakage, fire, etc. generated in lithium sulfur batteries using a liquid electrolyte are prevented.
Solid electrodes are classified into oxide based electrodes and sulfide based electrodes. The sulfide based solid electrolytes are generally used since the electrodes have higher lithium ionic conductivity and are stable in a broad voltage range, when compared with the oxide based solid electrolyte.
For example, in the related art, discharge capacity and lifespan of a battery were improved by using Li2S—P2S5 as a sulfide based solid electrolyte.
When the all-solid-state lithium sulfur batteries are manufactured, a contact area between the positive electrode active material and the solid electrolyte and a distribution pattern impact battery performance significantly. Accordingly, studies have been conducted to resolve the above problems by preparing a complex of the positive electrode active material and the solid electrolyte through mechanical milling.
Such all-solid-state lithium sulfur batteries include a complex electrode in which an electrode active material, a solid electrolyte and a conductive material are mixed. Since the size and the shape of each component of the complex electrode are different, the components are non-uniformly distributed in the complex electrode. Therefore, an interface between each component, particularly an interface between the electrode active material and the solid electrolyte, is not consistently formed, and thus, battery performance is deteriorated.